I agree, an argument for 4 is the fact that the hypervisor attack surface can be scaled up and down by adding/removing virtual devices. There is only a little set that stays permanently, like 30+ hypercalls on Xen. Overall compared to a standard OS interface (Linux has in the range of 350+ syscalls) this is still very little. The Solo5 VMM project tried even out another extreme by reducing the hypercalls to less than 10 if I remember correctly.